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books/docs/models/formula.md

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# Formula
Formulas are dynamic document properties that can be set on the fields
Forumlas are defined as javascript functions and are called whenever any property of the document is updated using the `set` method, or just before inserting or updating the document.
### Example
```js
module.exports = {
name: "FormulaExample"
fields: [
{
fieldtype: "Float",
fieldname: "a"
},
{
fieldtype: "Float",
fieldname: "b"
}
{
fieldtype: "Float",
fieldname: "c",
formula: doc => doc.a + doc.b
}
]
}
```
## Formula on child tables
In child tables, both the parent and the child records are passed
### Example
```js
{
fieldname: "amount",
fieldtype: "Float",
formula: (doc, row) => row.qty * row.rate
}
```
## Sequence
All child tables are executed first and all fields are executed in the sequence as defined in the DocType
## Fetching Values
You can use the `getFrom` function to fetch values from another document
Example:
```
{
"fieldname": "rate",
"label": "Rate",
"fieldtype": "Currency",
formula: (row, doc) => row.rate || doc.getFrom('Item', row.item, 'rate')
}
```
## Async
Forumlas are evaluated as promises, so you can either directly pass a value or a `Promise`